Electric furnace



S p 1935. E. HEDIGHER v 2,013,755

ELECTRIC FURNACE Filed lay 7. 1934 IN VENT OR.

ERNST HEDIQER.

ATTORNEY.

Patented Sept. 10, 1935 PATENT OFFICE ELECTRIC FURNACE Ernst Hediger,Niagara Falls, N. Y., assignor to The Globar Corporation, Niagara Falls,N. Y., a corporation of New York Application May '1, 1934, Serial No.724,268

3 Claims.

This invention relates to electric furnaces which are used in thepreparation of small amounts of metal alloys which it is desired toprepare in a high state of purity. Such alloys are needed for examplein-dental work where they are used as substitutes for gold. The highcost of gold and its strong color contrast to the tooth to be filledhave created a demand for metal substitutes and a need for carefulresearch on the physical and chemical properties of such substitutes.Small amounts of metal are also required for bearings under variouslubricating conditions. The selection of the alloy desired for a givenpurpose requires the preparation of a great number of samples for thevarious physical and chemical tests which are to be made in the courseof such researches.

My invention comprises in a concrete embodiment a combined heater andcrucible which is provided with a highly refractory protective coatingover the surfaces which come in contact with molten metal.

A specific embodiment of my invention is illustrated by the accompanyingdrawing in which:

Figure 1 shows a side elevation of my improved heater and crucible;

Figure 2 is a section on the line II-II of Figure 1;

Figure 3 is a section on the line III-III of Figure 1; and

Figures 4 and 5 are transverse sections of modifled forms of thecombined heater and crucible.

Referring to the drawing in detail, the semicylindrical heating element2 is composed principally of silicon carbide. Refractory bodies formedof bonded silicon carbide usually have a very high electrical resistanceespecially when cold and have a negative temperature coefficient ofresistance in the range in which they are operated. In my United StatesPatent No. 1,906,853, patented May 2, 1933, I have described a method ofmaking a silicon carbide resistor which has a low specific resistanceand a positive temperature coemcient of resistance in the principaloperating range, e. g. 550 C. to 1400 C. The raw mix for such a mixtureis made from a mixture of silicon carbide of different grit sizes towhich about 2 per cent of finely divided carbon is added. For exampleone fifth of the silicon carbide granules may range in size from 14 to36 mesh (per linear inch), one fifth from 50 to mesh, while theremainder are smaller than mesh. The mix is moistened with sodiumsilicate solution as a temporary agglutinant and made in the form oflong tubes. These tubes are dried and given a preliminary baking atabout 600 C. Before the final curing operation the tubes are given aprotecting coating produced by dipping them in a slip or slurry composedof finely divided sand and car bon suspended in water. The tubes arethen 5 placed in series in a bed composed of finely divided carbon andsilica which surrounds the tubes. The curing operation is effected bypassing a current through the tubes, using at first a high voltage tostart the current and reducing the 10 applied voltage as the specificconductivity of the tubes increases. The baked slurry may be removed bybrushing the tubes after the curing process. The ends of the tubes arethen trimmed to remove the terminal portions. The trimmed 15 tubes arethen cut in transverse and axial planes to yield semi-cylindricalelements of the desired length. The cutting of the tube can be performedby means of a silicon carbide orv fused alumina cut-off wheel of smallthickness (axially meas- 20 ured) e. g. inch thick. The central portionof any element can then be reduced in cross-sectional area to give theform indicated in Figures 1 and 2. This can be done by grinding orlapping the convex outer surface by means of a silicon 25 carbide orfused alumina wheel or by means of a wheel made of softer material towhich finely divided abrasive is supplied at the point of contact. Tomake the heating surface more chemically inert, the inside surface ofthe semi-cylindrical 30 element is painted with a slurry containing alarge percentage of alumina. This coating may consist for example of percent of alumina and 5 per cent of bentonite clay. This coating ishardened on the heating element by passing an electric current throughthe element. The coating is indicated by the reference character 3 inthe drawing. It helps to prevent contamination of the molten metal bythe silicon carbide element. The ends of the heating element rest interminal blocks 4 which are composed of carbon or graphite. These blocksare provided with openings extending therethrough but normally closed byaluminous plugs 5. The molten metal contacts therefore with thealuminous coating of the heating element and with the aluminous plugs inthe terminal blocks, the plugs being of such dimensions that they extendover the terminal areas of the semi-cylindrical trough. The molten metalmay be poured from the trough by removing one of the plugs. To 50facilitate the pouring, the heating element and terminal blocks aremounted on a base I. The carbon blocks 4 rest on metal plates 8 set inrecesses in the insulating base, The outer convex surfaces of theterminal portions of the element 55 2 I are sprayed with a thin coatingof conducting metal such as brass. This is conveniently performed bymeans a Schoop gun.

In assembling the heating element 2 within the carbon blocks 4 a carbonpaste can be used between each terminal portion of the element and theadjacent surface 01' the supporting block. This intermediate conductingmaterial fills in gaps caused by imperfect fitting oi the terminalportion of the heating element within the corresponding opening of theblock 4. The carbon paste can be made of graphite or lampblack mixedwith a small proportion of sodium silicate or a carbonaceous binder suchas glutrin (a residue from the sulphite treatment of wood pulp). Thecarbon blocks 4 can be set in direct contact with the metal plates 8 ora carbon paste can be added if any block does not fit well in its recessin 'the metal plate. Lead wires 9 are indicated for the connection ofthe heating element with an outside source of power.

The heating elements 2 can be made of carbon instead of silicon carbideor may be made oi. finely divided carbon bonded with clay or othermaterial which will increase the specific resistance of the element. 7

The inert coating can be made by using a slurry of zirconia mixed with asmall percentage (e. g. per cent) or bentonite.

Figure 4 shows a transverse section or a combined heating element andcrucible which has a cylindrical base. The area of cross-section can bediminished by grinding or lapping the outer convex surface as indicatedfor the semi-cylindrical element in Figure 2 where the outer surface ofthe element is applied to the periphery of an abrasive wheel whose planeof rotation is perpendicular to the axis of the element.

The central portion of the tubular element may be fluted as indicated inFigure 5 by means of an abrasive wheel which is rotated in a planeparallel to the axis of the element.

My heater and crucible have many advantages for research on metal alloyswhere large numbers oi! small batches of metal are required for testingpurposes. My apparatus is extremely compact. The heating elements can bemade at small cost and are very easily renewed. The conditions areiavorable to the production of alloys without contamination from othermetals.

Many variations may be made in the form of my compact melting furnaceand in the method of mounting it and connecting it to sources of power.The invention is defined within the compass of the following claims.

I claim: 4

1. A heating device for melting small quantities of metal comprising asemicylindrical trough composed mainly of silicon carbide which isbonded by passing an electric current longitudinally of the troughmember, metal terminals sprayed on the outer convex surfaces of the endsof the trough element, said element having a relatively thin wallintermediate of its two ends to concentrate the heating of an electriccurrent to a major extent in the intermediate portion or the troughelement; an aluminous coating attached to the inner surface of thetrough element, two carbonaceous blocks of comparatively large volumeprovided with grooves complementary to the semicylindrical metallizedterminals, and a packing of finely divided carbon between the metallizedterminals and the grooved surfaces of the blocks.

2. The method of making simultaneously a plurality of similar smallcrucibles which comprises molding finely divided silicon carbide into along hollow cylindrical element, heating the element by passing anelectrical current therethrough to bond the silicon carbide particles,baking an aluminous coating containing a small proportion of flux on theinside surface of the cylinder, cutting the hollow cylinderlongitudinally in an axial plane and also in a plurality of transverseplanes to obtain a plurality of semi-cylindrical troughshaped heatingelements, plugging the ends 01' each trough-shaped element with an inertreiractory material to make a plurality oi similar crucibles, andreducing the wall thickness of the intermediate portion of eachtrough-shaped element to concentrate a major portion of the heating insaid intermediate portion when an electric potential is applied to theterminals.

3. The heating device described in claim 1 in. which the crucible liningcontains more than nenety per cent oi? alumina.

ERNST HEDIGER.

